Direct Formation of Isotactic Poly(1-butene) Form I Crystal from

Article pubs.acs.org/Macromolecules

Direct Formation of Isotactic Poly(1-butene) Form I Crystal from Memorized Ordered Melt Fengmei Su, Xiangyang Li, Weiming Zhou, Shanshan Zhu, Youxin Ji, Zhen Wang, Zeming Qi,* and Liangbin Li* National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China ABSTRACT: Crystallization from memorized ordered melt (MOM) of isotactic polybutene-1 (iPB-1) is investigated with in situ Fourier transformation infrared microspectroscopic imaging and wide- and small-angle X-ray scattering. After being partially melted at high temperature, a small portion of form I crystal of iPB-1 recovers back when the temperature is lowered. This is different from a disordered melt, which crystallizes into form II directly. The recovery of form I crystal is attributed to the presence of MOM, which may keep the conformation order of form I crystal. Experimental evidence show that MOM possesses three characteristics, namely: (i) associating with the preserved form I; (ii) occupying only a small portion of the melt in the partially melted sample; (iii) being stable at high temperature. All this experimental evidence suggests that MOM locates at the boundary of crystal and melt. This is in line with the physical picture of nonclassical nucleation and growth theory or the multistage approach, where a partially ordered melt layer locates at the boundary of crystal and melt.

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transformation process.18−20 If the mesomorphic layer is a thermodynamic phase, as implied from the extrapolations of melting and crystallization temperature lines, this approach is still within the frame of CNT as interfaces among different forms are sharp. The multistage approach may fall in the frame of nonclassical nucleation theory (NCNT), if the “mesomorphic layer” is a diffusive partially ordered layer (DPOL) rather a thermodynamic phase.21−25 Comparing with CNT, a diffusive layer, instead of a sharp interface is proposed between crystal and melt in NCNT, which can be described with either simplified terrace function or density function theory.26−28 Though NCNT with DPOL may be close to the real physical picture in crystallization and melting, to determine the density profile in the DPOL is a grand challenge theoretically and experimentally. One can easily imagine the difficulty to directly probe the DPOL with thickness of tens of nanometers sitting between crystal and melt. Indirect approaches may have to be taken under current technological development. Memory effect in polymer crystallization may be an effective approach to demonstrate whether the diffusive layer with intermediate order exists between crystal and melt or not.29−32 In this approach, the existence of ordered structure that cannot be detected directly is manifested through induced crystallization after temperature is lowered down.33−39 If the diffusive layer exists between crystal and melt, it is expected to affect crystallization when lowering down temperature, and more

INTRODUCTION Crystallization and melting are the most common phase transitions in condensed matter, which control the community of material science and industry dominantly. The classic nucleation and growth theory is a powerful approach to describe crystallization and melting process, where a sharp boundary between crystal and melt is assumed and surface free energy is introduced as penalty to resist phase transition.1 For polymer crystallization, the Lauritzen−Hoffman theory (LH) takes the classic nucleation theory (CNT) as its foundation, where a sharp boundary between crystal and melt is also assumed.2−5 With a stem by stem approach and assuming a large lateral surface free energy σe, LH theory presents a simple physical picture to describe the formation of lamellar crystal of polymers. Though LH theory enjoys a great success in analyzing many experimental observations on polymer crystallization, critique and improvement have been taking place in the whole history since it was first proposed. Early criticisms from Frank et al.6,7 and Point et al.8−10 suggest that in LH theory crystal growth with stem length equal to lamellae thickness excluding stem thickening on growth surface is simplified and artificial. Nevertheless, introducing surface reorganization brings complications in the physics and mathematics treatment but leads to similar conclusions as LH theory. These debates are confined within the frame of CNT. Since 1990s, new experimental evidence has been reported, which stimulated a new flourishing period on the discussion of polymer crystallization.11−17 In later 1990s, Strobl proposed a multistage approach, where lamellar crystal forms through a melt-mesomorphic layer−crystal © 2013 American Chemical Society

Received: May 7, 2013 Revised: August 19, 2013 Published: September 5, 2013 7399

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microbeam X-ray source and Hi-star two-dimensional gas filled detector (Bruker).

importantly the structure formed by the DPOL should be different from that crystallized directly. To meet this special requirement, isotactic polybutene-1 (iPB-1) with unique phase behavior can be a right candidate.40−42 iPB-1 normally crystallizes into form II from bulk melt in quiescent condition, which is a metastable state with 11/3 helix geometry and a tetragonal unit cell.43,44 The melt grown form II spontaneously transforms to form I at room temperature, which is stable with 3/1 helix geometry.40−42,44−47 Cooling down a partially molten iPB-1 with form I crystals, memory effect may lead to three different crystallization behaviors: (i) crystallized into form I through an epitaxial growth from the remaining form I crystals; (ii) crystallized into form II without any memory effect; (iii) the DPOL (if it does exist) crystallizes into form I, while the remaining melt crystallizes into form II as normal melt does. In this work, systematic experiments are designed to study how memory effect of form I affects subsequent crystallization. In situ Fourier transformation infrared (FTIR) imaging, in situ wide and small-angle X-ray scattering (WAXS and SAXS) measurements support the existence of the diffusive layer with intermediate order in between crystal and melt.

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RESULT The effect of Th on memory effect of form I is first investigated. After heating at Th (128, 130, or 132 °C) for 5 min, the samples are cooled to room temperature naturally. Figure 1a depicts

EXPERIMENTAL SECTION

Material and Sample Preparation. iPB-1, a semicrystalline homopolymer, is kindly supplied by Basell (PB0800M). The melt flow index is 200 g/10 min (190 °C/2.16 kg, ISO 1133), and the numberaverage (Mn) and weight-average molecule weight (Mw) are about 25 and 77 kg/mol, respectively. iPB-1 films only containing form I crystal used for the study were prepared with following procedure. First, iPB-1 melt was quenched to 90 °C to induce the formation of form II spherulite after removal of thermal history, during which the growth of spherulite was monitored with polarized optical microscopy (POM). When the size of spherulites reached about 150 μm, the films were rapidly quenched in ice water. A great number of tiny spherulites or lamellae formed surrounding the giant spherulites, which prevents the further growing of these giant spherulites. Unlike previous study,48 hedrites were not observed, probably because of different temperature used. Afterward, the samples were left at room temperature for a month to ensure completely transformation from form II to I.45,47,49,50 Experiment and Measurements. For the study of memory effect, three different high temperatures (Th), 128, 130, and 132 °C, were chosen to partially melt the original samples with form I. For the convenience of description, we name the samples as S128, S130, and S132, respectively. After keeping at Th for different times (th), the samples which were sandwiched by two ZnSe windows (for FTIR measurments) or Kapton films (for WAXS and SAXS measurements) were cooled to different low temperatures (Tl) to lead iPB-1 crystallized. The whole experimental process was followed with in situ FTIR imaging, WAXS or SAXS. In situ FTIR imaging measurements were performed at infrared spectroscopy and microspectroscopic imaging beamline at National Synchrotron Radiation Laboratory of China. The beamline equips with a Bruker HYPERION 3000 microscope coupled with Bruker IFS 66v FTIR spectrometer. With a 64 × 64 elements focal plane array (FPA) detector, 250 × 250 μm2 areas can be measured simultaneously. Every element detects an area of 3.9 × 3.9 μm2. The measured spectrum wavenumber range is 3900−700 cm−1 with a resolution of 4 cm−1 which can be measured in 3.5 min for 128 scans. The baseline of spectrum was carefully adjusted uniformly using OPUS 5.5 package. The strong absorption bands at 905 and 923 cm−1 are characteristic peaks of forms II and I, respectively, whose intensities are used to represent the content of each crystal form.51−55 Height of peak was considered as integral intensities of the conformational bands in all of IR data in this study. WAXS and SXAS experiment was performed using an in-house setup with an Iμs

Figure 1. (a) Optical microscopy images of the sample during the heating process. (b) IR single spectrum inside (point I) and outside (point O) the giant spherulite at different temperatures. (The scale bar in the image is 50 μm.)

optical micrographs of iPB-1 sample with form I during the heating process to 128 °C, from which it can be seen that tiny spherulites start melting at around 120 °C prior to melting of the giant spherulite. The different melting temperatures of matrix and giant spherulite provide a well-defined system, where the study of memory effect is focused on the giant spherulite and the matrix serves as a reference. Thanks to FTIR imagining with high spatial resolution, it is possible to obtain FTIR spectra at different locations simultaneously. Figure 1b displays spectra inside and outside the spherulite, which are signed with point ‘I’ and ‘O’ in Figure 1a, respectively, at different temperatures during heating and cooling process with Th of 128 °C. From Figure 1b, it can be seen that the original sample only contains form I. When increasing temperature close to melting point, the intensity of 923 cm−1 (I923) band decreases. Outside spherulite, 923 cm−1 band disappears at about 120 °C, while I923 inside spherulite only drops part of its initial value at room temperature. When cooled down the temperature, the I923 inside spherulite recovers partially back again, which suggests that the content of form I increases back. This observation is different from normal melt crystallization of iPB-1, during which only form II forms.40,44,46,49 In contrast, outside spherulite 923 cm−1 band does not recover back. Instead, only the 905 cm−1 band representing form II, appears. This suggests that recovery of form I at low temperatures is related to the survived form I crystals. Taking the I923 and I905 (intensity of 905 cm−1 band), it is possible to reconstruct the spatial distribution of forms I and II at different temperatures. Figure 2 shows the distributions of I923 and I905 at different temperatures during heating and cooling processes of S128. Outside the spherulite, the I923 band almost disappears when temperature reaches 128 °C during heating and does not appear again during cooling process. 7400

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Figure 2. 2D images of intensity distributions of the characteristic bands of forms I and II of S128.

Figure 3. 2D images of intensity distributions of the characteristic bands of forms I and II of S130.

Figure 4. 2D images of intensity distributions of the characteristic bands of forms I and II of S132.

However, the I923 band inside the spherulite still exists at 128 °C though the intensity has decreased already. It can increase back partially when cooling down the temperature. Figure 2 also shows the evolution of I905 with temperature. In the heating stage there is no form II over the whole sample, while in the cooling process form II crystal formed both inside and outside the spherulite. Moreover, large portion of the melt recovers back into form I rather than crystallizes into form II, as demonstrated in Figure 2. This creates a low valley of form II inside the giant spherulite region. Similar experiments were also made with samples S130 and S132. The distributions of the two samples are shown in Figures 3 and 4. It can be seen from Figure 3 that only a little content of form I preserved at 130 °C. The recovery of form I decreased in comparison with S128, though the recovery is still obvious when lowered the temperature. For S132, the form I is almost melted at 132 °C as shown in Figure 4. There is no obvious increase of I923, meanwhile I905 shows an almost homogeneous

distribution inside and outside the spherulite, which is very different from that of S128. The recovery ratio (CI) of the three samples (S128, S130, and S132) is presented in Figure 5 quantitatively, where CI =

c Th I 923 ̅ − I 923 ̅ i Th I 923 ̅ − I 923 ̅

× 100%

c Ii9̅ 23, ITh 9̅ 23, and I9̅ 23 are averaged value of I923 over the whole giant spherulite at 120 °C where only form I large spherulite exists, melting temperature Th and crystallization temperature (45 °C) respectively. As can be seen in Figure 5, with Th of 128 °C, the CI is about 56%, indicating that memory effect leads to significant recovery of form I. And CI decreases with increasing Th. The averaged intensity of form II over the whole giant spherulite (I9̅ 05) is also shown in Figure 5. It can be seen that higher Th corresponds to higher I9̅ 05 of form II when the temperature is lowered.

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Figure 5. Quantitative recovery ratio (CI) of form I and the averaged intensity of form II (I9̅ 05) inside spherulites of S128, S130, and S132 after lowering the temperature to 45 °C.

Figure 7. Normalized absorption intensity of 923 cm−1 during the cooling process after being kept at 128 °C for 90 and 5 min, respectively.

How does iPB-1 crystallize into form I directly from melt? The above results clearly show that recovery of form I requires preservation of form I crystals at Th. One possibility is that the preserved form I crystals in the melt act as nuclei for the recovered form I to grow. In order to check this, S128 samples are cooled to different temperatures Tl for isothermal crystallization. Experiments with three different Tl, 37, 80, and 100 °C, were carried out, respectively. Figure 6 plots the

sample which is kept at 128 °C for 5 min. Evidently, keeping the sample at Th does not affect the memory effect. As 90 min is far longer than the terminal time of polymer chain, the MOM is a stable structure at 128 °C. As form I is normally obtained through the transformation from form II, one may wonder whether the recovery of form I is also via a transforming process of melt-form II-form I. An experiment with high Tl of 105 °C is designed to exclude this process. The FTIR absorption intensity distributions of forms I and II at Th of 128 °C are presented in Figure 8, parts a and b, respectively, which show that part of form I preserves inside the giant spherulite and no form II exists in the whole region. Figure 8c shows the corresponding I923 immediately after cooling down to Tl of 105 °C. An obvious increase of intensity is observed inside spherulite, while no any trace of form II is detectable. Figure 8d presents I905 of form II after isothermal crystallization at 105 °C for 146 min. The optical pictures of the sample at Th and after crystallization at 105 °C for 146 min are presented in Figure 8, parts e and f, respectively. Interestingly, form II appears around the giant spherulite rather than inside spherulite. A POM picture of this sample after isothermal crystallization for 350 min is given in Figure 8g, which shows clearly that form II grows around the partially melted spherulite. Here FTIR imaging study gives three observations, which exclude the possibility of the transformation approach for the recovery of form I from partially melted samples. (i) Recovery of form I and crystallization of form II occur at different regions, respectively. (ii) Recovery of form I takes place earlier than form II appears. (iii) Even after the formation of form II around the spherulite, no observable transformation occurs within the experimental time (350 min) at this temperature. Indeed, it is widely reported that the transformation rate from form II to I slows down as temperature increases, which may be eventually stopped at high temperatures.45,47 Clearly the recovery of form I and the crystallization of form II happen independently in both space and time. This can be attributed to independent sources for them to form, namely MOM for form I and normal disordered melt for form II. Isothermal crystallization at lower temperature (90 °C) was also carried out after melting at 128 °C. From Figure 9, it can be seen clearly that form II formed inside the spherulite after quenching to 90 °C. Comparing the results in Figures 8 and 9, it demonstrates that lower temperature favor formation of form II from melt inside the spherulite. The recovery of form I through memory effect is further confirmed by in situ WAXS and SAXS study. An example with Th of 125 °C is shown in Figure 10 during cooling at different

Figure 6. Normalized intensity of 923 cm−1 as a function of crystallization time at different Tl during the cooling process of samples with Th of 128 °C.

normalized I923 against crystallization time. The intensity is normalized for the convenience of comparison during which we set the intensities at Th (128 °C) and at 80 °C during heating (before melting) to be 0 and 1, respectively. It is obvious that I923 of form I reaches its plateau value immediately when the temperature reaches Tl, although different Tl values recover different amounts of form I crystals. Further extending crystallization time does not lead to continuously growth of form I, though large amount melt still exists. With Tl at 100 °C, the recovery of form I is less than 5% and 95% of polymer chains melted from initial form I still remain in molten state. Clearly this 95% polymer melt cannot grow into form I through form I nuclei existing here, which is in line with the general observation that form I cannot grow from melt directly. Obviously, only a small portion of the “melt” which memorized the structure of form I can transform back to form I at Tl. For the convenience of description, we name the melt capable of transforming back to form I as “memorized ordered melt” (MOM). Is the MOM stable at Th for a long time? Or does it die off and relax down to a normal disordered melt for a long time? The sample was kept at a Th of 128 °C for 90 min. The I923 remains a constant of about 0.77 during melting at 128 °C for 90 min. When cooling these samples to Tl, 53% form I recovers back (Figure 7), which is almost the same with that of the 7402

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Figure 8. Absorption intensity of forms I (a, I923) and II (b, I905) after heating the sample to 128 °C. The absorption intensity of forms I (c) and II (d) after isotherm crystallization at 105 °C for 146 min. Corresponding optical pictures (a, b; c, d) are shown (e and f, respectively). Polarizing optical microscope image of the sample after isothermal crystallization at 105 °C for 350 min (g).

from 125 °C, the diffraction intensity of form I increases. This confirms the recovery of form I directly from the MOM. On the other hand, the appearance of form II happens until temperature reaches about 91 °C, which is far delayed compared to the recovery of form I. Figure 10b presents a quantitative estimation of crystallinities of two crystal forms. The recovery of form I follows a continuous increasing process from 28% to 34%, while crystallization of form II occurs sharply below 100 °C and results in a crystallinity of about 30%. Similar as that observed in Figure 8, the crystallinity evolutions of forms I and II run independently without interference. As shown in Figure 10b, though form II starts to crystallize at lower temperature, it reaches a plateau of crystallinity at higher temperature (about 80 °C). On the other hand, form I shows a continuous increase down to about 50 °C. This suggests crystallization of form II cannot take the MOM as its source, though melt crystallization favors growing form II. The full width at half-maximum (fwhm)) of form I (220) diffractions is plotted in Figure 10c against temperature during cooling. A sharp decrease of fwhm occurs from 125 °C to about 90 °C, which is followed by a gentle further decrease. This twostage decrease of fwhm coincides well with the recovery of form I, which suggests that recovery of form I is accompanied by an increase of the lateral size of lamellar crystals. Whether there were other factors such as formation of new lamellae leading to the recovery of form I existing or not were also checked. SAXS is used to directly measure the long period of lamellae. It can be seen from Figure 10d that when only form I forms during cooling from 125 to 95 °C, the long period remains almost constant. Only when form II crystallizes during cooling from 95 to 70 °C, obvious decrease can be seen. Therefore, it can be concluded that the change in the long period is due to the formation of form II lamellae.

Figure 9. 2D images of intensity distributions of the characteristic bands of forms I and II of sample during heating from 120 to 128 °C and quenching at 90 °C respectively.

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DISCUSSION On the basis of the FTIR imaging, WAXS and SAXS measurements on the heating and cooling processes of iPB-1, some conclusions are drawn from the results. (i) Memory effect on the recovery of form I is related to the preserved form I at Th. If no form I is preserved, no recovery of form I occurs. This is supported by the results with different Th as well as the spatial distribution of form I during heating and cooling (Figures 1−4). (ii) The MOM occupies only small volume of melt, which can transform back to form I. The majority of melt

Figure 10. (a) Selected integrated 1D diffraction profiles and (b) crystallinity of forms II and I versus temperature during the cooling process (the number in part a indicates temperature and the unit is °C). (c) Full width at half-maximum of form I (220) diffractions during the cooling process. (d) Evolution of the long period from SAXS during the cooling process.

temperatures, where a rate of 5 °C/min is applied. Figure 10a gives several representative integrated one-dimensional diffraction curves. Immediately after the cooling program starting 7403

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loses their memory of form I at Th and crystallizes into form II at Tl. The latter is in line with general observation on melt crystallization of iPB-1 (see Figures 2, 5 and 8). (iii) The MOM is stable, which does not vanish with time th at Th (see Figure 7). (iv) The recovery of form I is not via form II as an intermediate state (see Figures 8 and 10). Our experimental evidence support a direct transition from the MOM to form I. (v) The recovery of form I is accompanied by an increase of lateral size of lamellar crystal (see Figure 10). An interesting question is where the MOM locates. Experimental results show that the presence of survived form I is a precondition for the recovery of form I from the MOM. This suggests that the MOM bounds close with survived form I crystal,. Taking this as the criterion, two possible locations may accommodate the MOM, which are illustrated in Figure 11.

described in NCNT, which is more likely an intrinsic feature of melting and growth of crystals.

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CONCLUSION The unique phase behavior of iPB-1 is used to study the memory effect, which helps us to unveil whether a diffuse ordered layer exists between crystals and melt. After form I crystals of iPB-1 partially melt at high temperatures, FTIR imaging, WAXS and SAXS show that a small portion of form I crystals recover back when temperature is lowered. As disordered melt can only crystallize into form II, the recovery of form I in partially melted samples is attributed to the presence of memorized ordered melt (MOM), where conformational order of form I is possibly preserved. The presence of MOM is associated with the survived form I and only takes a small portion of total melt, which indicates that MOM may locate at the boundary of form I crystal and melt. This picture is consistent with nonclassical nucleation theory, and may also in line with the multistage crystallization approach proposed by Strobl.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (Z.Q.) [email protected]. *E-mail: (L.L.) [email protected].

Figure 11. Schematic illustration on the location of memorized ordered melt (MOM).

Notes

The authors declare no competing financial interest.

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One sits in the middle of lamellae; another locates between crystal and melt. These two cases are essentially originated from the same physics, where crystalline order imposes constraint on melt around the crystals. The only geometric difference is that symmetric and asymmetric constraints are applied on the two cases, respectively, which may only affects the layer thickness of the MOM. As the MOM is generated after partially melting and is stabilized by survived form I, the layer of MOM fits well with the physic picture of DPOL in NCNT. Note melting and crystallization are two reversible counter phase transitions. The MOM generated in partial melting is expected to have the same structure as that in DPOL during crystallization. The second question is, what kind of order exists in the MOM? The MOM or DPOL is an exclusive source for the formation of form I, which refuses to grow into form II even the experimental conditions favor the later to occur. This suggests that the MOM may possess ordered structure close to form I. Forms I and II have intrachain conformational order of 3/1 and 11/3 helices, respectively, while at interchain level, they pack with hexagonal and tetragonal units, respectively.41,42,44 Partial melting may erase either intra or inter chain order. Generally, inter chain positional order is relatively fragile to destruction, while inter chain orientational order like liquid crystal order and intra chain conformational order can survive at high temperature. As the MOM is due to the constraint from form I crystalline order, we speculate that orientation and 3/1 helix conformation may be partially preserved in the MOM, similar as our recent observation at the growth front of isotactic polypropylene.56,57 The MOM is rather stable, which shows no relaxation within a time far longer than the terminal time of polymer chains. Nevertheless, as direct thermodynamic information is absent, we are not sure whether the MOM is a thermodynamic metastable phase at Th. If the MOM is a mesophase, it is in line with the model proposed by Strobl. Otherwise, the MOM can be the DPOL

ACKNOWLEDGMENTS The authors thank Prof. Bernard Lotz for fruitful discussion. This work is supported by the National Natural Science Foundation of China (51033004, 51120135002, 51227801, 51303166, U1232128) and the 973 program of MOST (2010CB934504). The research is also in part supported by the China Postdoctoral Science Foundation (Grant No: 2012M521233) and the Opening Project of Soochow University Biomedical Polymers Laboratory. The experiments were carried out in the National Synchrotron Radiation Lab (NSRL).

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dx.doi.org/10.1021/ma400952r | Macromolecules 2013, 46, 7399−7405